The other way to distinguish black holes depends on the redshift of signals from black holes. The astrophysical black holes forms after the death of Population III stars which are the first generation stars. Their observational signals exist at \(z < 20\)  [1]}. In contrast, the redshift of PBHs goes through the whole history of the universe after the big bang. From this point, the signals from black holes with \(z > 20\)  should come from the PBHs. In detecting high redshift signals, gravitational wave is a powerful detection channel which carries information to travel through cosmological distance due to weak interaction between gravity and matter. Since the first gravitational wave event from the merger of binary black holes [2]}, the binary black holes system is the important gravitational wave source, so the detection of gravitational waves from PBH binaries is significant, which can bring us high redshift signals.
